Rapid evaporative ionization mass spectrometry (“REIMS”) is a technology which has recently been developed for the real-time identification of target substances, for example for the identification of biological tissues during surgical interventions. REIMS analysis of biological tissues has been shown to yield phospholipid profiles having high histological and histopathological specificity.
Coupling of REIMS technology with handheld sampling devices has resulted in iKnife sampling technology, which can provide intra-operative tissue identification. This technology allows surgeons to resect target tissues, such as tumours, more efficiently by providing intra-operative information that can assist a surgeon in minimizing the amount of healthy tissue removed whilst helping to resect the target tissue.
In a known iKnife sampling system, a target substance is subjected to alternating electric current at radiofrequency which causes localized Joule-heating and the disruption of the target substance along with desorption of charged and neutral particles. The resulting aerosol is then transported to a mass spectrometer for on-line mass spectrometric analysis. The mass spectrometric analysis generates one or more mass spectra for the aerosol, which are then analysed so as to classify the target substance.
Correct classification of target substances can be extremely important for determining courses of action. For example, when it is desired to resect unhealthy tissue in a human subject, it is important that sufficient unhealthy tissue is correctly identified and removed and that sufficient healthy tissue is correctly identified and left behind.
REIMS technology can also be used by non-surgical operators in non-surgical procedures to identify target substances ex vivo or in vitro. Correct classification of target substances is also highly desirable in these non-surgical procedures.
Reference is made to N. Strittmatter et al., Anal. Chem. 2014, 86, 6555-6562 which discloses an investigation into the suitability of using rapid evaporative ionisation mass spectrometry as a general identification system for bacteria and fungi.
The known approach for analysing bacterial colonies by rapid evaporative ionisation mass spectrometry involves using bipolar electrosurgical forceps and an electrosurgical RF generator. A bacterial colony is scraped from the surface of an agar layer using the bipolar electrosurgical forceps and a short burst of RF voltage from the electrosurgical RF generator is applied between the bipolar electrosurgical forceps. For example, it is known to apply 60 W of power in a bipolar mode at a frequency of 470 kHz sinusoid. The RF voltage which is applied to the electrosurgical forceps has the result of rapidly heating the particular portion of the bacterial colony which is being analysed due to its nonzero impedance. The rapid heating of the microbial mass results in an aerosol being generated. The aerosol is transferred directly into a mass spectrometer and the aerosol sample may then be analysed. It is known to utilise multivariate statistical analysis of spectrometric data in order to help distinguish and identify different samples.
It is desired to provide improved methods of spectrometric analysis in order to classify aerosol, smoke or vapour samples.